This specification relates to sweep-free stimulated Brillouin scattering-based fiber optical sensing.
FIG. 1(a) shows a diagrammatic representation of a Brillouin scattering process. Brillouin scattering is a nonlinear process in which acoustic phonons 120 (that have a frequency ω0 and are associated with a propagating medium) either spontaneously scatter a forward propagating optical wave 100 (that has a frequency ωL and is called “pump”) into a backward propagating wave 140 (that has a frequency ωS and is called “probe”), or mediate, via a stimulated interaction, power transfer between counter propagating (pump and probe) waves. In either case, the backward propagating probe light 140 has a characteristic Brillouin frequency shift (ω0=ωL−ωS from that of the pump), which varies with many types of changes in the propagating medium, such as temperature and mechanical stress. Therefore, this Brillouin frequency shift (BFS) can provide information on the surrounding temperature and strain distributions along an optical fiber.
Some Brillouin scattering-based sensors rely on the stimulated Brillouin scattering (SBS) process in which two counter-propagating pump and probe waves generate acoustic waves in an optical fiber, which then transfer optical power from the pump to the probe if the latter frequency is downshifted from that of the pump by the BFS. In some Brillouin scattering-based sensors, the evolution of temperature/strain induced BFS is determined from consecutive recordings of the whole Brillouin gain spectrum (BGS). FIG. 1(b) shows that the BGS can be measured 150 by sweeping the optical frequency of the probe (or the pump) over the entire BGS. The BGS is a Lorentzian-shaped spectrum, having a width of −30 MHz (FWHM) at 1550 nm in a standard single mode optical fiber (SMF), and is measured as densely as required by a specific application. Consequently, such implementations cannot typically be used in resolving fast, dynamic changes in the measured fiber. In addition, a typical sensitivity of the BFS measurement can be 1 MHz/° C. and 500 MHz/(1% strain). Since the probe signal can be weak when long (tens of kilometers) fibers are interrogated, averaging over multiple measurements is required at each frequency point, thereby further slowing down the frequency scanning rate and the overall measurement speed.